Alloy for high temperature tooling applications

ABSTRACT

Thermal fatigue is the predominant mechanism that limits the service life of dies in semi-solid forming of steels since the feedstock to be shaped has a paste-like character. A novel alloy, more resistant to these conditions than any other alloy, has been developed. Eutectic carbides in Stellite alloys are replaced in this novel alloy with molybdenum-rich intermetallic compound particles between dendrites. This novel alloy offers at least 3 times longer service life with respect to Stellite 6 alloy that has been tested under conditions that mimic the steel semi-solid forming process and has been identified as the most suitable. The exceptional performance of the novel alloy is attributed to its outstanding resistance to oxidation and to softening at elevated temperatures and to its cobalt based matrix free from the hard and brittle carbides that have a negative impact on crack growth process.

Tool materials that can withstand the process conditions are employed in forming operations. The tool materials to be used in tooling for high temperature forming operations must withstand the high temperatures, the mechanical and thermal loads encountered at these temperatures, oxidation and loss of strength. The present invention offers a new tool material for the semi-solid processing of steels, an attractive near-net shaping process that has not been commercialized until today due to a lack of suitable die materials.

The die materials intended for semi-solid forming of steels must withstand thermo-mechanical cycles at elevated temperatures, wear and oxidation. The tool material described by the present invention offers a three-fold improvement in the service life with respect to that provided by cobalt-based alloys that have been the material of choice and widely used until today. Hence, it is a very attractive material for the manufacturers of forged steel parts. The material of the present invention, whether employed as the die material or as a hardfacing alloy, offers a very significant opportunity for semi-solid processing of steels, an attractive near-net shaping process that has not been commercialized until today due to a lack of suitable die materials.

STATE OF THE PRESENT ART

Tooling issue is a very critical one since tool assemblies are responsible for one fifth of the part cost in plastic forming operations [1]. Dies employed in semi-solid forging of steel parts are among tooling that are faced with the most severe conditions [2-5]. Stresses originating from thermal cycles are essentially the major life-limiting factors since the mechanical stresses are limited owing to paste-like features of the feedstock to be shaped in the forming dies [6]. Hot work tool steels are not at all suitable for tooling to be employed in semi-solid forming of steels in spite of the fact that they are very attractive from a cost standpoint [7-15].

Cobalt-based alloys are attractive candidates for high temperature tooling applications owing to an exceptional performance in a wide temperature range [16-22]. Stellite 6, a popular cobalt-based alloy which can be used as the die material or as a hardfacing alloy, has been shown with the present art to offer a three-fold improvement in tool service life with respect to hot work tool steels [23, 24]. However, the hard and brittle carbides dispersed in between the ferrite dendrite arms have played a key role in crack growth and led to an increase in the crack growth rate. Additionally, carbides were found to be more susceptible to oxidation in high temperature wear tests and have impaired the wear resistance of cobalt-based Stellite alloys by increasing the wear rate [25]. The high cost of cobalt based alloys is the major impediment that limits their wide spread use.

DETAILED DESCRIPTION OF THE INVENTION

A novel cobalt based alloy has been developed in the present invention to be employed in the semi-solid shaping of steels. This novel alloy has been designed without carbon with a consideration of the negative impact of the carbides in stellite alloys on crack propagation under thermal cycles encountered at elevated temperatures. Another critical feature of the alloy of the present invention that is different from its counterparts is its much higher iron content (up to 12 wt %).

This compositional adjustment is intended to account for the dilution of the hardfacing layer with iron from the underlying steel substrate during hardfacing operations.

Another motivation for this change is to provide a substantial cost reduction in the cobalt based alloy.

The alloy of the present invention does not contain carbon. Besides, its iron content is much higher than its counterparts. This feature not only improves the cost aspect but also accounts for the dilution effect often encountered in hardfacing of tool steels.

Owing to the chemical composition of the present alloy, the eutectic carbides typical of stellite alloys are replaced by molybdenum-rich intermetallic compound particles dispersed at dendrite boundaries.

The alloy of the present invention offers a three-fold improvement in the service life of dies with respect to those manufactured from stellite 6 alloy that has been identified to be the best die material tested until today under semi-solid steel forming process conditions. This exceptional performance of this novel material is attributed to its resistance to high temperature oxidation and to temper softening as well as to a cobalt-rich matrix phase free from the hard and brittle carbides that impair the crack growth resistance.

Carbon-free cobalt based alloy of the present invention was prepared by melting commercial purity elements in a 2 kg capacity induction furnace under vacuum. The alloy melt thus obtained was subsequently cast into boron nitride coated permanent mould between 1580-1600° C. under vacuum.

The microstructure of the alloy of the present invention consists of a ferritic dendritic solid solution matrix phase and molybdenum-rich intermetallic compound particles at interdendritic sites (FIG. 1).

Carbides in stellite alloys are replaced in the alloy of the present invention with molybdenum-rich intermetallic compound particles. The hardness of these particles is nearly two times higher than that of the matrix phase. The hardness of the matrix and the molybdenum-rich particles were measured to be 382±52 HV and 700±143 HV respectively.

Samples of the present alloy, 25 mm×25 mm×20 mm in size, were submitted to thermal fatigue testing in order to identify their performance. The minimum and the maximum temperatures of the thermal cycle were selected to be 450 centigrade and 750 centigrade, respectively, with a consideration of the die surface temperatures encountered in the conventional forging and semi-solid forging operations. The former temperature is the temperature to which the die is heated to in plastic forming operations while the latter is the highest temperature measured at the surface of the die during semi-solid forging of steels. The front face of the thermal fatigue test sample was heated to 750 centigrade within 30 seconds with an oxyacetylene torch and subsequently cooled to 450 centigrade within the next 30 seconds with forced air blow (FIG. 2). The temperature of the front and rear faces of the thermal fatigue test samples were measured with K-type thermocouples fixed into 3 mm diameter holes drilled at 0.1 mm from the respective surfaces. Thermal fatigue tests were terminated as soon as oxidation author thermal fatigue cracks were detected on the front face. The damage introduced with thermal cycling were evaluated qualitatively with optical and stereo microscopes.

The lowest and the highest temperatures at the rear face of the thermal fatigue sample were measured to be 486 and 580 centigrade, respectively, when the temperature of the front face was cycled between the minimum and maximum temperatures of 450 and 750 centigrade. These temperature differences between the front and rear faces have set up thermal gradients across the section of the samples. The temperature difference between the front and rear faces of the sample is as much as 192 centigrade within 27 seconds of the start of a typical thermal cycle. The maximum tensile and compressive stresses acting on the front face of the thermal fatigue samples were estimated to be 472 MPa and 210 MPa, respectively, with a consideration of the thermal gradients measured across the section of the samples.

The front face of the thermal fatigue sample prepared from the alloy of the present invention faces the highest temperatures and thus the highest thermal stresses during thermal cycling and is shown in FIG. 3. The thermal fatigue resistance of this sample revealed no signs of thermal fatigue cracking, until 13000 thermal cycles and was thus identified to be exceptional. Several minor lines linked with the blistering and wrinkling of the surface oxide film were detected on the front face during the regular checks performed every 500 cycles after a total of 13000 thermal cycles (FIG. 3 a). It has taken these minor traces another 3000 thermal cycles to develop into legitimate thermal fatigue cracks (FIG. 3 b). Finally, a surface crack of considerable size has formed by the spallation of the surface oxides at the intersection of two surface wrinkles after a total of 16000 thermal cycles (FIG. 3 b). The thermal fatigue test was terminated at this point and the front face of the sample was prepared for investigation with standard metallographic practices. The surface crack was found to go through the grains instead of through the grain boundaries (FIG. 3 c).

Thermal fatigue test had to be terminated after 5000 cycles when the hot work tool steel sample coated with Stellite 6 alloy hardfacing tested under exactly the same conditions. Thermal fatigue cracks had already traversed the 2 mm thick stellite 6 surface layer entirely at this point. Thermal fatigue cracking was found to start after 4500 cycles and the thermal fatigue crack was found to grow nearly 2 mm during the next 500 cycles during the regular 500 cycle inspections [24].

It takes 13000 thermal cycles for thermal cracking to start and another 3000 cycles for the thermal fatigue crack that has formed to grow only 1 mm. (FIG. 4).

This simple comparison evidences that the crack growth rate in the alloy of the present invention is at least 10 times lower than that in the Stellite 6 alloy. It is fair to claim from the foregoing that the alloy of the present invention offers not only an outstanding thermal fatigue crack initiation but also to thermal fatigue crack growth resistance. This exceptional crack growth resistance of the alloy of the present invention is attributed to its microstructure free of brittle carbides that were shown to encourage crack propagation in Stellite 6 alloy [24, 26].

The potential of the alloy of the present invention under thermal cycling conditions at elevated temperatures is appreciated more when the performance of the hot work tool steels under the same conditions are considered. Hot work tool steels could withstand the same conditions only for 1000 thermal cycles due to surface degradation linked with severe oxidation [23, 24]. Deep cracks filled with voluminous oxides were noted on the front face of the hot work tool steel sample when the thermal cycling was terminated after 15000 cycles.

Stellite 6 alloy that has been identified to be the most successful high temperature alloy until today, whether employed as the die material itself or as weld overlay surface layer, could withstand these cycling conditions for only 5000 cycles without cracking on the front face [23]. It is thus fair to claim that the alloy of the present invention offers at least 3 and at least 10 times longer service life with respect to Stellite 6 alloy and hot work tool steel, respectively.

The outstanding thermal fatigue performance of the alloy of the present invention evidences its exceptional resistance to high temperature oxidation. This oxidation resistance is attributed to chromium in the alloy composition that oxidizes preferentially and forms a slowly growing, stable and protective oxide film on the surface [23, 24]. The presence of such an oxide (Cr2O3) film on the surface was confirmed with low angle x-ray diffraction analysis. This stable and protective Cr2O3 layer served as a barrier to excessive oxidation under thermal cycling test conditions. The much lower tendency of the alloy of the present invention to oxidation with respect to hot work tool steels is evidenced also by thermo gravimetric analysis and by the metallographic analysis of sections of samples submitted to thermal cycling. The surface oxide of the alloy of the present invention is only 3-4 μm thick after 16000 thermal cycles, much thinner than the oxide on the surface of hot work tool steel samples, measured to be nearly 50 μm in after only 500 thermal cycles. Cr2O3 film on the surface is claimed to be resistant to thermal stresses and retains its integrity.

Another feature that imparts to the alloy of the present invention its high thermal fatigue resistance is its resistance to loss of strength at elevated temperatures. Resistance to softening is a key feature required in tool materials to ensure adequate resistance to crack initiation as well as to crack growth. A hard alloy protects its surface oxides and the surface oxides, in turn, protect the underlying die steel. Surface oxide films offer this protection by resisting against plastic deformations produced by mechanical and/or thermal stresses, against blistering, expansions and finally to cracking and spallation. Hence, surface deterioration and damage are avoided and cracking is thus delayed.

The resistance of the alloy of the present invention to loss of strength at elevated temperatures has been confirmed with hardness measurements across the section of the samples submitted to thermal cycling. The softening in the present alloy is very small and is confined to the surface during the first couple of thousand cycles (FIG. 5). The average hardness measured to be 560 HV before thermal cycling, is still 500 HV after the first 5000 cycles. This hardness level implies a much higher softening resistance of the present alloy with respect to hot work tool steels, the hardness of which is measured to be as low as 250 HV after the same number of thermal cycles [23]. Hardness of the alloy of the present invention has continued to decrease steadily with increasing number of thermal fatigue cycles. However, the hardness of the alloy of the present invention even after 16000 thermal cycles is 400 HV, and is thus higher than the hardness measured in Stellite 6 alloy after 5000 cycles. It is fair to conclude in view of the foregoing that the high resistance to softening of the alloy of the present invention makes a favourable impact on its high thermal fatigue resistance.

DESCRIPTION OF FIGURES

FIG. 1: Microstructure of the alloy of the present invention: (a) dendritic matrix phase, (b) Molybdenum-rich intermetallic phase.

FIG. 2: Change in temperatures measured on the front and rear faces of the sample during a typical thermal cycle.

FIG. 3: Surface blistering detected on the front face of the thermal fatigue test sample of the cobalt-based alloy of the present invention after 13000 thermal cycles (a), (b, c) development of this surface blister into a thermal fatigue crack after 16000 cycles. (a and b) scanning electron and (c) optical microscope micrographs.

FIG. 4: Section view of the thermal fatigue crack after 16000 thermal cycles.

FIG. 5: Change in hardness across the section of the thermal fatigue sample with number of thermal cycles.

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1. A cobalt based alloy with no carbon and 9-15 wt % iron in its chemical composition.
 2. An alloy according to claim 1, with a ferritic dendritic matrix phase and molybdenum-rich intermetallic particles at dendrite boundaries.
 3. An alloy according to claim 1, the extent of softening after 5000 cycles in thermal fatigue test of which, is less than 15%.
 4. An alloy according to claim 1, containing 0.72 wt % Si, 1.58 wt % Mn, 29.22 wt % Cr, 5.50 wt % Mo, 3.77 wt % Ni, wt % 11.50 Fe, the balance being Co. 